An optical fiber interferometric rate gyro comprises principally a laser source, a device for splitting, and recombining the light, a single-mode wound optical fiber, a detecting device and a device for processing the detected signal.
The wave from the laser source is split into two waves directed towards both ends of the fiber so that one of these two waves is propagated in the direction of the rotational speed, the other being propagated in the opposite direction.
The light intensity detected after recombination of the two waves emerging from the two ends of the optical fiber is the result of interferences created between the two waves received. The phase shift between these two waves is directly tied to the rotational speed, SAGNAC effect, and this phase shift is calculated, from the detected intensity, by the processing device.
In conventional devices, the intensities of the two waves emerging from the two ends of the fiber are assumed to be linearly connected to the intensity supplied by the source, the coefficients of proportionality being constants which depend on the elements of the optical device whose characteristics are invariable in time. To get around the variations in the power of the light emitted by the source, and so the variations in power of the two waves received, the detected intensity is controlled from the real value of the emitted light power, and the phase shift connected to the value of the rotational speed is then determined from this single detected intensity resulting from interferences between the two emerging waves.
In practice, in the optical fiber interferometric rate gyro, the intensities of the two waves emerging from both ends of the optical fiber are not only connected to the incident light power but may also depend on couplings in the fiber, on the diffusion and on the mode of propagation in this fiber, these different factors varying in time and creating variations in intensity of the two waves emerging from the fiber cause variations in the light intensity detected which do not correspond to speed variations.
It is not possible to take these variations into account in the coefficients connecting the emerging intensities with the incident intensity because the conditions of propagation in the fiber vary in time.